Plasma burner with adjustable constriction structure in gas flow path



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July 21, 1970 G. HESS 3,521,106

PLASMA BURNER WITH ADJUSTABLE GONSTRICTION STRUCTURE IN GAS FLOW PATH Original Filed April 8. 1966 nited rates Patent 3521,16 Patented July 21, 1970 3,521,106 PLASMA BURNER WITH ADJUSTABLE CONSTRIC- TION STRUCTURE IN GAS FLOW PATH Gunther Hess, Erlangen, Germany, assignor to Siemens Aktiengesellschaft, Erlangen, Germany, a corporation of Germany Continuation of application Ser. No. 541,300, Apr. 8,

1966. This application May 28, 1969, Ser. No. 831,262

Claims priority, applicgtigi Germany, Apr. 12, 1965,

Int. (:1. H01j 1/50, 1/88, 17/26 US. Cl. 313-146 4 Claims ABSTRACT OF THE DISCLOSURE This application is a streamlined continuation of application Ser. No. 541,300, filed Apr. 8, 1966, now abandoned.

My invention relates to plasma burner for heating gases in an electric arc chamber in which an arc rotates between electrodes.

In a plasma burner of this type, a continuous gas flow is heated in an electric are so that the enthalpy of the gas is increased. If the gas is given a high flow velocity by means of a Laval nozzle, jet propulsion drives can be constructed and wind tunnels can be provided, for example, in accordance with this principle. Moreover, the high gas temperatures also permit their direct use for example for chemical reactions.

Plasma burners with a rotating arc are particularly suitable for transforming high power because local overheating of the electrodes, which can be accompanied by vaporization of the electrode material, is thereby prevented. A relatively great electrode surface can be adequately cooled by water if the foot or base of the arc is rotated with a high velocity over the electrode surface. This can be achieved by applying a magnetic field whose field lines extend perpendicular to the arc. Rotating arcs are used particularly when electrodes of heavy metals such as tungsten cannot be inserted in a chamber which contains aggressive working gases but, rather, metals of low melting points such as copper are to be employed therein.

With plasma-burners, and particularly with those having rotating arcs, the problem occurs of passing the greatest possible portion of the supplied quantity of working gas through the arc zone and mixing the entire working gas thereafter as much as possible. Basically, the are can be made to extend radially between coaxially located cylindrical electrodes or the arc can extend in an axial direction between cylindrical electrodes of the same size which are located axially opposite one another. With both of these known constructions, the are is traversed by the gas stream in a direction substantially perpendicular thereto. K

It is often necessary with plasma burners that the power transformed in the burners, or, in other words, the length of the are be varied during the operation thereof. The invention is based on the fact that with increased electrode spacing and constant gas throughput, the lower gas flow velocity permits the arc to assume a larger cross section which opposes the increased power output. If, on the other hand, the gas throughput is increased in order to maintain the prevailing flow velocity in the arc zone, then a larger quantity of gas must be heated. Both effects oppose the increase in temperature.

It is accordingly an object of my invention to effec-= tively control the power of the plasma burner by providing a substantially constant spacing of the electrodes and to also maintain very constant the middle quantity of gas passing through the rotating arc zone, which is indefinitely spread out in width. With the foregoing and other objects in view, I provide a plasma burner whose electrodes, in accordance with the invention, form a conical annular gap providing a narrow constricted passage in the gas supply means to the arc, the conical annular gap being adjustable in width by means of an adjustment drive. The working gas is thereby conducted for a relatively long period obliquely through the arc. It is of particular importance that the foot of the arc is blown out from the central electrode onto the cylindrical electrode in the gas flow in dependence on the width of the conical annular gap between the electrodes, whereby the power transformed in the arc chamber is variable with constant gas throughput. Also, operating instabilities broaden in a desired manner the width of the path of the are on the cylindrical electrodes.

In my copending application Ser. No. 541,223, filed Apr. 8, 1966, and now Pat. No. 3,456,146, I have provided a plasma burner with two electrodes located fixed with respect to one another and so constructed that a central electrode forms With an essentially cylindrical electrode a conical annular gap in the gas supply to the are. This arrangement however has the material disadvantage that the are power can only be controlled through the gas throughput.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

While the invention has been illustrated and described as plasma burner, it is not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the present invention and within the scope and range of equivalents of the claims.

The invention, however, together with additional objects and advantages thereof, will be best understood from the following description when read in connection with the accompanying single figure of the drawing showing an axial cross section of the plasma burner constructed in accordance with the invention.

In the figure there is shown a cylindrical electrode 1 having a conical extension 2 in which a central electrode 3 having a conical shoulder extension 4 is fitted. An angle of between the conical wall and the cylinder axis 8 has been found to be particularly suitable. The

- central electrode 3 has a dish-shaped construction facing the arc chamber 5 and has an annular edge 6 from which the are 7 extends to the cylindrical electrode 1. The conical annular gap between the electrode extensions 2 and 4 merges into a widened substantially cylindrical ring 9 having one or more supply tubes 10 for supplying a working gas which lead in a tangential direction into the interior of the ring 9. The central electrode 3 has a holder 11 which can be axially adjusted in a neck 12 as indicated by the double-headed arrow 13. An annular excitation coil 14 coaxially surrounds the arc chamber 5 and when the excitation coil 14 is energized, the axially extending field lines cause the are 7 to rotate. The magnetic field thereby has an effect particularly on the foot or base of the arc which describes the greater rotary path.

Cooling slots are provided in the cylindrical elec trode 1 which can be formed of a material having good heat conductivity though not magnetically conductive, such as copper. The slots 15 are sinuous or meandershaped, as shown in the figure, and are connected through an annular distributor 16 and an annular collector 17 with a coolant inlet 18 and a coolant outlet 19 in a coolant circuit or loop not otherwise illustrated. In the simplest case, water can be used as the coolant. The cylindrical electrode 1 is threaded by means of the screws 20 to the extension 2 consisting of brass. The cylindrical electrode 1 is electrically insulated with respect to the extension 2 by means of an insulating body 21 having a pot-shaped peripheral surface and by any well known conventional means for insulating screws 20 from cylindrical electrode 1. Ceramic or a material having a mica base as well as temperature-resistant plastic materials such as polytetrafluoroethylene can be employed as insulating material. After the screws 20 are loosened, the cylindrical electrode 1 can always be easily exchanged if desired. An annular gap 22 supplements the electrical insulation of the extension member 2. Working gas penetrating into the annular gap 22 is greatly cooled and forms a relatively good insulator. The insulating material 21 is provided so that it is optically covered or shielded against direct heat rays of the arc and of the heated gas. This is of great importance because electrically insulating material, in general, is a poor heat conductor and cannot therefore be sufiiciently cooled when it is exposed directly to the arc radiation or beam. Coolant channels 23 are provided in the extension member 2. Tubes 24 and 25 provide a connection. with a coolant loop. A hollow cylinder 26, for example also of brass, in connected, for example by soldering or brazing, to the extension 2 provided with a conical shoulder. The wall of the hollow cylinder 26 defines with the central electrode 3 a gas distribution space in the ring 9. A neck 12 is electrically insulated from the hollow cylinder 26 by means of an insulating ring 27 of the cross-sectional shape shown in the figure, and is gas-tightly connected therewith. The connection can be effected by means of screws 29 which can be protected by tubular sleeves of insulating material.

The outer casing 30 slides in the neck 12 and is located on and coaxial to the holder 11 of the central electrode 3. The casing 30 can consist of a material such as brass and is electrically insulated by an insulating layer 31 from the current carrying holder 11. A coolant can be supplied to the central electrode 3 in a double tube system of the illustrated type. Through a ring 32 of angular cross section, consisting of brass for example, the electrode ring proper can be well cooled by providing high flow velocity in a narrow gap. The electrode ring and dish-shaped connection can be formed of a material having good heat conductivity such as copper. The coolant can be supplied through the inner tube 33 to the holder 11v and removed through the collector channel 44 in the head member 45. An electric potential is simultaneously applied to the head member 45 from the voltage source 46 for the arc 7. By means of a set screw 47, which is clamped around. the holder 11 by the clamping cheeks 48, the central electrode 3 can be adjusted in the axial direction. The conical gap between the shoulder extensions 2 and 4 can accordingly be widened or narrowed. The tube 10 for the gas supply communicates tangentially with the distributor chamber of the ring 9 so that the twist thereby applied to the working gas as it enters the ring chamber 9 supplements the arc rotation. Since the conical gap between the extensions 2 and 4 leads directly to the arc, twisting of the gas supply at such high velocities is achieved that a magnetic field for producing rotation of the arc can be dispensed with. altogether. This can be of particular advantage in those cases where weight or energy sources must be economized. On the other hand, with strong magnetic fields, the quantity of gas passed through the arc zone can be increased by inserting the supply tube 10 into the ring chamber 9 in a direction opposite to that in which the arc rotates. The advantage derived therefrom is that higher gas temperatures can be achieved.

The annular excitation coil 14 is energized by an A.-C. generator 49. It produces a substantially axial magnetic field in the arc chamber 5. It is understood of course that if stronger magnetic fields are required or the magnetic fields are to act upon a more distant region of the arc, two annular coils can be employed for producing a cusp field in the arc chamber 5.

I claim:

1.. Plasma burner for heating gas to plasma-forming temperature in an arc chamber comprising a pair of members spaced from one another, means for rotating an arc between said members, one of said members comprising a cylindrical electrode and an extension thereof having a conical surface electrically insulated from said cylindrical electrode, and the other of said members comprising an axially adjustable central electrode having a conical surface and fitted within said extension, said conical surfaces defining therebetween a conical annular gap forming a constriction in the path of gas supplied to said arc, adjusting means for varying the width of said conical annular gap, and supply duct means for supplying the gas to the arc chamber, said supply duct means extending in a direction tangential to the rotational di-= rection of said arc.

2. Plasma burner according to claim 1, wherein the arc chamber is substantially cylindrical and the walls of the conical annular gap and the axis of the cylindrical arc chamber form an angle of 60 therebetween.

3. Plasma burner according to claim 1, wherein said means for rotating an arc between said members is a magnetic field producing means surrounding said arc chamber, said conical annular gap being located outside the arc chamber and being of such width as to increase the flow velocity of the gas to blow said are into the arc chamber in lieu of applying a magnetic field thereto.

4. Plasma burner according to claim 1, wherein said conical annular gap merges into a widened substantially cylindrical annular chamber, said supply duct means comprising a plurality of tubes communicating with and extending tangentially to said annular chamber for introducing the gas to be heated therein.

References Cited UNITED STATES PATENTS 3,004,189 10/1961 Giannini .n 31323l X 3,201,560 8/1965 Mayo et a1 313231 X 3,222,569 12/1965 Winzeler et al. a- 313231 X 3,226,592 12/1965 Cough et al. 313161 X 3,359,734 12/1967 Ferric et a1. 313231 X FOREIGN PATENTS 693,580 9/1964 Canada. 969,831 9/1964 Great Britain.

JAMES W. LAWRENCE, Primary Examiner P. C. DEMEO, Assistant Examiner US. Cl. X.R. 

